Pu heat preservation material of polyurethane insulation
Pu heat preservation material of polyurethane insulation
< P> as of the end of 2001, China's solar water heater's about 3 3 million m2, 70% of the world's ownership, the annual output up to 7. 8 million m2, annual sales of product was 10 times higher than in Europe, was the first use of the solar water heater, and producer.
One of the biggest market share vacuum tube solar water heater is a genuine Chinese products, from design development to production are not can be drawn lessons from foreign experience, there is a lot to be gradually perfect in practice.
As is known to all, for the solar water heater, especially the vacuum tube solar water heater is very important for the quality of polyurethane thermal insulation layer, it determines the heat preservation effect of the water heater.
Products polyurethane products has always been a 'three points, seven distribution of bubble', that is to say, not only need good polyurethane foaming materials, foam need a more scientific process and strict operation procedure.
Bubble contraction is the most common problem in solar water heater, it can lead to deformation of the tank, summon the stare blankly and craze, namely solar manufacturers often said 'peeling', 'shell', 'take a spine' wait for a phenomenon.
To explore the causes of bubble contraction below.
2 2 bubble contraction causes analysis.
1 form bad bad bubble foam is one of the main causes of bubble contraction, its size shrinkage rate is 2% ~ 10%.
At present, the solar water heater with polyurethane foam is one or more pouring free foaming, give priority to with multiple pouring.
Foaming process, except under atmospheric pressure are not subject to any external pressure.
Polyurethane foam filling the space before the arrival of gel process must be completely filled with gel, if the bubble volume continues to expand, the formation of the bubble, bubble hole will significantly elongated, severe cases can be a fibrous, its physical and mechanical performance is very poor, this is called 'bubble'.
It cooled, severe contraction due to bubble hole in the gas condensate, heated bubble hole gas expansion and inflation.
Multiple casting foaming in such a way that increases the proportion of bad 'bubble'.
Furthermore a border of adjacent two casting forming foam, due to the difference of shrinkage when you apply, easy to cause bubble craze.
When designing foam molding process, therefore, should be designed in a way pouring excessive filling of foaming.
Excessive filling, refers to the foam completely fill the space before the gel, gel after due to limited space, the bubble volume can not continue to swell, and produced by foaming pressure, beneficial to form the round the structure of the spherical bubble hole, this form of foam bubble hole with high intensity, good stability, not easy to deformation.
And the solar energy water heater of foaming process on the gel after volume expansion can not control, because the tank could not excessive filling of foaming under pressure.
If you want to bubble to normal performance, it is necessary to the structure design and foaming process of solar water heater to make a big change, otherwise can not fundamentally solve the problem.
2 foam density from the theory of control, CFC -
11 as the foam density of the foaming agent in arrived at 32 kg/m3, the foam should have sufficient strength to resist the change in temperature.
But considering the solar water heater is bad work environment, work inside and outside temperature difference (
May reach 100 ℃)
, and the working life of the requirements, we recommend bubble lowest density shall be not less than 35 kg/m3.
If the foaming agent is as
141 b, then foam density should be increased to more than 38 kg/m3.
Some foaming materials suppliers, in order to improve its market competitiveness and the foam density dropped to below 30 kg/m3.
That although on the surface to reduce the foam production cost, but low foam strength, weight is not enough to carry water tank full of water, making bubbles were squashed slowly, some of the weight of the water tank is supported by the top of the vacuum tube, vacuum tube with the risk of being crushed.
Precedent for this kind of phenomenon in the portion of solar energy production manufacturer.
3 than the deviation of the vast majority of foaming machine material than fixed 1:1, also inconvenient adjustment, but the actual than common foaming material viscosity, temperature and the influence of such factors as deviation from 1:1.
This is from the machine free bubble with manual free bubble density difference roughly.
By hand free bubble density as A benchmark, when the free bubble density is lower than manual free bubble machine, suggests that A material (
This bubble color is white, low intensity, feel is soft, easily when the temperature is low.
When the free bubble density higher than that of manual free bubble machine, show that B (
This bubble deep color, high strength, feel is hard and brittle.
These cases should be immediately try to check the material ratio, check to see if the filter clogging, pressure, temperature indicating whether normal, ensure the accuracy of A and B material proportion.
For low pressure casting machine because of its low pressure, only 5 ~ 7 kg/m3, when compressed air to the lower limit of pressure fluctuations or foaming material viscosity slightly larger, will affect the mixing effect, coefficient of A and B material mixed uneven, light bubble color is darker, bubble pore is bulky, bubble brittle;
Or bubble uneven color, irregular spots or pattern.
This is because the local overproduction of A material (
, or local materials caused by excessive (B
Result in the crack of the bubble is easy)
4 environmental temperature of polyurethane foam and the influence of temperature relationship is very large.
Because the foam < / P>